Approximately 60 percent of the U.S. population is overweight or obese, and obesity has been linked to numerous health problems. Physical activity has been recommended for overweight and obese adults to enhance weight- loss and/or weight-maintenance, and to reduce risk of chronic disease, including heart disease and cancers of the breast and colon. Despite the benefits of exercise, only 20 percent of overweight or obese adults meet the minimum national recommendations (30 min/day, 5 days/week), and drop out in the first few months of exercise programs is as high as 50 percent. Given these data, there is a need to improve adherence to exercise programs, especially for overweight and obese adults. Consistent with prevailing theoretical models, exercise promotion interventions typically focus on cognitive (e.g., self-efficacy) and social (e.g., social support) factors, but do not typically target affective processes. Alternatively, Hedonic Theory posits that continuation of a behavior is guided by how one feels as a consequence of performing the behavior. Consistent with Hedonic Theory, we have recently shown that enjoyment of physical activity at baseline of an exercise promotion program predicted intervention success. Moreover, recent research examining affective responses during exercise has shown that individual bouts of moderate intensity exercise (e.g., a brisk walk) can lead to acute negative affective responses among overweight and sedentary adults, potentially making exercise adherence more difficult. Alternatively, when overweight adults choose their own walking pace, it results in a more positive affective response than when a specific intensity is prescribed. Therefore, in this study we will examine the effects of self-paced walking compared to prescribed moderate intensity walking on adherence to otherwise identical, six-month walking programs among overweight or obese individuals. We hypothesize that recommending self-paced intensity exercise for overweight adults will result in better adherence to the program (i.e., more minutes of exercise) than prescribing exercise at moderate intensity, as recommended in public health guidelines. If our hypothesis is confirmed, the results will inform future exercise recommendations among sedentary, overweight and obese adults. Additionally, consistent with Hedonic Theory, we will explore acute affective responses to exercise as a predictor of adherence to the exercise program using ecological momentary assessment (EMA;i.e., electronic diaries). If affective responses to exercise, assessed in real time in participants'natural environments, are shown to predict adherence to the walking program, this will signal the need for greater attention to affective variables in health behavior models and as targets of health promotion interventions. PUBLIC HEALTH RELEVANCE: For overweight and obese adults, self-paced exercise may be more pleasurable and easier to maintain than exercise prescribed at moderate intensity (e.g., brisk walking), thus leading to better adherence and thereby better health outcomes. In this study we will compare adherence rates to self-paced exercise versus prescribed moderate intensity exercise among sedentary, overweight and obese adults. We will also ask participants to rate their feelings during each exercise session, using handheld electronic diaries, to determine whether these feelings are related to adherence to the walking program.